Field of the Invention
The present invention relates to a radio communication method and apparatus capable of being connected to various types of data terminal equipment.
With the recent great advance in computer and information processing technology, many personal computers come to have the capability of communication with other computers or information processing systems. For such communication, media such as ISDNs (integrated services digital networks) or LANs (local area networks) are now widely available.
Various radio communication systems have been developed to satisfy a need for mobile communication capability or a need to realize communication without using troublesome cables. To meet such requirements, radio communication systems using an SS (spread spectrum) technique or an optical beam technique are now in practical use.
When communication is performed via radio waves, there is a possibility that a plurality of terminal equipment start transmission at the same time. In this case, collisions occur among data transmitted by the terminal equipment and thus the data is destroyed without being correctly transmitted to receiving equipment. During a transmitting operation, sending equipment cannot receive any data and thus cannot know whether the transmitted data has been destroyed.
One conventional technique to avoid the above problem is CSMA/CA (carrier sense multiple access with collision avoidance). In this technique, terminal equipment sends a carrier signal serving as a request-to-send signal several times during a collision detection window before starting a transmission operation thereby notifying other terminal equipment of its intention to start transmission of data. The carrier signals are transmitted at random intervals determined by random numbers so as to avoid collision with other terminal equipment. The terminal equipment monitors carrier signals transmitted by other terminal equipment during the collision detection window, and if it is concluded that no other terminal equipment is transmitting a carrier signal indicating a request of starting its data transmission, then the terminal equipment starts its data transmission.
If the terminal equipment detects a carrier transmitted by another terminal equipment during the collision detection window, the terminal equipment transmits a carrier a predetermined number of times and waits for a time period determined by a random number. In a subsequent collision detection window, the terminal equipment transmits a carrier signal so as to notify its intention to start data transmission. In this technique, it is determined whether there is another terminal equipment desiring to start data transmission by detecting a collision of carrier signals, thereby avoiding the collision of data.
The conventional CSMA/CA technique described above, however, has a significant problem that even when some terminal equipment has important data that should be transmitted immediately, the terminal equipment has to wait if another terminal equipment is transmitting a carrier signal indicating its intention to start data transmission.